The present invention is directed to improved optical imaging systems using polarized light. Many animals, such as fish, have visual systems that may exploit optical polarization. Some biological systems are believed to compute difference signals using parallel arrays of photoreceptors which are optimally tuned to orthogonal polarizations. It is believed that polarization-differencing systems can improve the visibility of objects in scattering media by serving as a common mode rejector and differential amplifier which reduces the effect of background scattering and which amplifies the signal from a target whose polarization-difference magnitude is distinct from the background.
Optical scattering caused by suspended particals (e.g. fog, rain, plankton) has been shown to diminish the visual contrast of objects. Although polarization sensitive vision is well documented in aiding navigation, some types of polarization-sensitive vision also may serve to enhance the visibility of targets in scattering media.
The human eye is a highly evolved efficient visual mechanism for everyday viewing conditions. Typical viewing conditions are those which exist in relatively clear air with sufficient light intensity. However, there are many times when viewing conditions are not optimal for human vision, such as in foggy, cloudy, and underwater environments where particulates are present. Human beings cannot efficiently detect polarization and polarization differentials.
As noted above, there are species of animals which have demonstrated polarization sensitivity. These species include bees, some salamanders and certain types of fish. It is believed that bees use polarization as a navigational aid. It is not known how species of fish, like the Lepomis cyanellus (green sunfish) may utilize such polarization information. But, it has been hypothesized that these fish might actually subtract polarization intensities at two orthogonal polarizations.
The human visual system computes the intensity of visible light at discrete points on the retina without utilizing polarization information. An analogous image is created in the lab by taking a "summed" or non-polarized (NP) image. To produce this image, the camera operator measures a frame through a linear polarizer aligned along a unit vector e1, then adds to it another frame taken through the same linear polarizer aligned along e2, a unit vector perpendicular to e1.
After the two frames are taken and added, the result is the final non-polarized image. A PD image can also be created using a camera and a linear polarizer. By taking frames at two orthogonal polarizations oriented in the same way as in the NP image, then subtracting the images instead of adding, a PD image can be formed. At each point, the intensity can be represented as: EQU .sub.pd I(x,v)=I.parallel.(x,y)-I.perp.(x,y) EQU .sub.np I(x,y)=I.parallel.(x,y)+I.perp.(x,y)
A number of prior art systems have been disclosed in the literature which either discuss or disclose optical polarization systems and theory. In the Sep. 12, 1991 Edition of "Nature", Cameron and Pugh discussed the existence of double cones as a basis for a new type of polarization vision in vertebrates in which it was postulated that certain invertebrates and vertebrates are sensitive to light polarization.
In "Polarization Imagery", 112 Optical Polar Imagery (1977), Walraven discussed the polarization of reflected radiation providing useful information that can be used in remote sensing applications to help distinguish different natural surfaces with similar structural signatures. Dr. Gary D. Gilbert and J. C. Pernicka of the Systems Development Department of the U.S. Naval Ordinance Test Station has disclosed a system in which underwater targets were illuminated, photographed and photometered both with and without the use of a circular polarization technique.
Wolff, Mancini, et al. developed a liquid crystal polarization camera which combined CCD camera technology with liquid crystal technology to create a polarization camera capable of sensing the polarization of reflected light from objects at pixel resolution.
In "Differential Polarization Imaging", December, 1987 Journal of the Biophysical Society, Kim, Keller and Bustamante discussed a theory of differential polarization imaging. It was shown that for any arbitrary object, images can be obtained by combining different incident polarizations of light measuring the specific polarization components transmitted or scattered by the object.
Cheng, Chao, et al. in Multi-Spectral Imaging Systems using Acousto-Optic Tunable Filter, (1993) discuss activities in the development of a new type of remote sensing multi-spectral imaging instrument using acousto-optic tunable filter as a programmable band pass filter. The remote sensor filter provides real time operation and observational flexibility in the measurement of spectral, spacial and polarization information using a single instrument.
Finally, U.S. Pat. No. 3,992,571 to Garlick et al. discloses a detector for measuring differential optical polarization effects which comprises a television camera incorporating a polarization analyzing system. The camera generates two similar video signals representing two views of the same scene which are derived by means of like components-differing only in respect of their polarization characteristics. The two video signals are compared over the whole scene on a point-by-point basis.
None of the prior art systems disclose a system for enhancing vision using polarization differences and removing common modes. It is believed that a man-made polarization difference imaging (PDI) system, similar to that believed to be found in some types of polarization-sensitive animals, could enhance the visibility of target features in scattering media. It would be desirable to have an optical enhancement system which would provide enhanced vision using polarized light differentials. Such a system could be used to enhance vision underwater, in fog and through clouds. It would be particularly desirable to provide a system including a mechanism for automatically rotating the polarizer to optimize the polarization differentials. These and other objects of the present invention will become apparent from the following summary and detailed description.